Lately, nitride semiconductor light emitting devices such as a blue light emitting diode (LED) or a laser diode, using nitride semiconductor, have been in practical use. On the other hand, a ZnO based compound is superior to a GaN based compound (which means a GaN or a compound in which a part or all of Ga of GaN is substituted with other element of group III element and the same applies hereinafter) in emitting light in a range of a short wavelength. Concretely, an exciton of ZnO, which is formed by recombination of a hole and an electron in a solid, is stable even in a room temperature because of having a large binding energy of 60 meV (GaN has that of 24 meV). Further, in GaN based compound, although light is emitted with high efficiency by using an InGaN based compound (which means that a mixed crystal ratio of In and Ga can be varied variously and the same applies hereinafter) by adding In, the efficiency is lowered accompanied with decreasing of In. It is said in case of InGaN based compound that it is unnoticeable for crystal defects, because portions of low potential arise partially by variation in a concentration of In and carriers are captured there. On the contrary, when In decreases, it is understood that the crystal defects become noticeable since the concentration of In is unified and especially, portions where carriers are easily captured do not arise. Such problem does not occur in ZnO based compound even the wavelength is shortened, though the InGaN based compound becomes more disadvantageous to shortening the wavelength. Of course, GaN itself or AlGaN based compound (which means that a mixed crystal ratio of Al and Ga can be varied variously and the same applies hereinafter) becomes disadvantageous to further shortening of the wavelength.
A light emitting device having a structure shown in FIG. 7 is known as a light emitting device using a ZnO based compound (cf. for example PATENT DOCUMENT 1). In FIG. 7, on a sapphire substrate 31, a buffer layer 32 made of ZnO, and an n-type contact layer 33 made of n-type ZnO are formed, and thereon, a light emitting layer forming portion 38 is formed by laminating an n-type clad layer 34 made of MgZnO based compound, an active layer 35 made of CdZnO based compound, and a p-type clad layer 36 made of MgZnO based compound. Further a p-type contact layer 37 made of Zno is deposited, and a part of the n-type contact layer 33 is exposed by etching a part of a semiconductor lamination portion. Thereafter, an n-side electrode 39 is formed on an exposed surface, and a p-side electrode 40 is formed on a surface of the p-type contact layer 37.    PATENT DOCUMENT 1: Japanese Patent Application Laid-Open No. 2002-94114    PATENT DOCUMENT 2: WO01/73170A1